Presentation of hypoxemia during OLV is multifactorial. It depends on the operation side (OLV better in left thoracotomies), on preoperative lung function [best indicator = Partial Pressure of Oxygen in Arterial Blood (PaO2) divided by Fraction of Inspired Oxygen (FIO2)] and on the distribution of perfusion. The PaO2/FIO2 is the best parameter to describe the full spectrum of ventilation-perfusion abnormalities, independent of the inspired oxygen concentration level.